Modern cable telecommunications systems are typically built with a Hybrid Fiber Coaxial (HFC) network topology to deliver services to residences and businesses. By using Frequency Division Multiplexing, multiple services on these systems are carried on Radio Frequency (RF) signals in the 5 MHz to 1000 MHz frequency band. The HFC topology carries the RF signals in the optical domain on fiber optic cables between the headend/hub office and the neighborhood, and then carries the RF signals in the electrical domain over coaxial cable to and from the home. The fiber optic signals are converted to and from electrical RF signals in a device called a fiberoptic “node.” In the coaxial portion of the network, the signal is split to different housing areas and then tapped off to the individual homes. The RF signals continue to be transported through the home on coaxial cables and connected to devices in the home. Due to attenuation in the coaxial cable and split/tap losses, “RF amplifiers” are used periodically in the coaxial plant to amplify the electrical signal so they are at an acceptable level to be received by the devices at the home.
Information is transported from the headend/hub office to the home, such as video, voice and internet data, over the HFC network. Also, information is transported back from the home to the headend/hub office, such as control signals to order a movie or internet data to send an email. The HFC network is bi-directional, meaning that signals are carried on the same network from the headend/hub office to the home, and from the home to the headend/hub office. The same coaxial cable actually carries the signals in both directions. In order to do this, the frequency band is divided into two sections, “forward path” and “return path”, so there is no interference of signals. The “forward path” or “downstream” signals, which typically occupy the frequencies from 52 MHz to 1000 MHz, originate in the headend or hub as an optical signal, travel to the node, are converted to electrical RF in the node, and then proceed to the home as electrical signals over coaxial cable. Conversely, the “return path” or “upstream” signals, which typically occupy the frequencies from 5 MHz to 42 MHz, originate in the home and travel over the same coaxial cable as the “forward path” signals. The electrical signals are converted to optical signals in the node, and continue to the hub or headend over fiber optic cables.
The HFC network is capable of carrying multiple types of services: analog television, digital television, video-on-demand, high-speed broadband internet data, and telephony. Cable Multiple System Operators (MSOs) have developed methods of sending these services over RF signals on the fiber optic and coaxial cables. Video is transported using standard analog channels which are the same as over-the-air broadcast television channels, or digital channels which are usually MPEG2 signal over a QAM channels. The most common method for carrying data services, telephony services and sometimes video, is Data-Over-Cable Service Interface Specification (DOCSIS). In order to transport information on RF signals, the MSOs have a significant amount of equipment that converts the services so they can be carried on RF signals. Examples of this equipment would be Cable Modem Termination Systems (CMTS), QAM modulators, Upconvertors and Digital Access Controller (DAC). Also, devices in the home are required to convert the RF signals to signals that are compatible with television sets, computers and telephones. Examples of these devices are television set-top boxes, cable modems and Embedded Multimedia Terminal Adapter (EMTA). These devices select the appropriate forward path signals and convert them to usable signals in the home. These same devices also generate the return path signals to communicate back to the headend/hub office. MSOs have a significant investment in the equipment at the home and headend/hub offices that utilize DOCSIS and similar protocols. They also have a significant network operation investment to manage this type of network with regards to maintenance and customer service.
Today, the MSOs are facing competition from traditional telecommunication companies. These companies are utilizing new technologies where fiber optic cables are laid very close to the home, called Fiber-to-the-Curb (FTTC), or all the way to the home, called Fiber-to-the-Home (FTTH). With these technologies, many more services and higher quality can be delivered to the homes, while also lowering the maintenance cost of the network because the active components are reduced. A common type of FTTH network is Passive Optical Network (PON) where no active components exist between the headend/hub/central office and the home. There are several types of PON's including Broadband PON (BPON) and Gigabit-capable PON (GPON) which are actively being deployed by telecommunication companies in the United States. The technical standard for the BPON is defined in ITU-T Recommendation G.983 and for the GPON is defined in ITU-T Recommendation G.984. For the sake of this disclosure, the GPON will be used as the reference since this is the latest PON architecture being actively deployed, but this invention can apply to other forms of PONs.
FIG. 1 shows a typical architecture for a GPON and FIG. 2 shows a typical ONT for a GPON. As illustrated in FIG. 1, a forward path of a typical GPON network contains headend 1 with a broadcast transmitter 4 and optical amplifier 6, and a wave division MUX/deMUX 8, which provides communication to a 1×n optical coupler 9 at node 10 over optical fiber 3 to couple n homes 12 to the communication signal. At the home 12, an Optical Network Termination unit 11 (ONT) converts the optical forward signals via optical triplexer 14 containing receivers 15 and 17 and transmitter 16. Interface module 13 provides the Ethernet signals to Ethernet output 19 for internet data, the POTS signals to RJ11 twisted pair wires 18 for telephone, and broadcast signals to coaxial cable output 20 for television (if the video overlay is used). In the return path, the ONT converts the Ethernet input and RJ11 twisted pair to an optical baseband digital signal. Any television return signals utilize the Ethernet input. At the headend/hub/central office, the GPON utilizes the OLT 2 system as the interface between the PON and network-side.
Instead of using DOCSIS and similar protocols like an HFC network, the GPON utilizes baseband digital protocol for forward path and return path signals. The forward path baseband digital signals carry internet data, telephony and sometimes television service by using Internet Protocol (IPTV). The GPON also has an option for a forward overlay wavelength to provide enhanced services to the home. Often, the overlay wavelength is at 1550 nm and delivers video services in the forward path using Frequency Division Multiplexing just as the HFC network. This overlay wavelength is shared over many homes, up to 10000. Unlike the HFC Network though, the only option for return signals on the GPON is using the baseband digital return signal. Because of the method that information is transported, the GPON utilizes vastly different equipment at the home and headend/hub/central office 1 compared to HFC network.
MSO's cannot utilize their current methods of transporting information over a PON, and therefore cannot utilize their current headend/hub equipment and home devices in this architecture. In order to compete with the telecommunication companies, MSOs would like to migrate to FTTH networks, such as GPON, to offer perceived and real increases in services and quality. MSOs have a very large investment in DOCSIS and similar equipment at the headend/hub office and the home, which cannot be utilized in a GPON network. Also the network management systems for maintenance and customer service are built around DOCSIS equipment and, therefore, running a second system in parallel would be costly.
Technical issues exist for utilizing the MSO's current infrastructure equipment in a GPON network. For example, the GPON network cannot provide sufficient, cost-effective forward bandwidth per home for targeted, unique narrowcast services if they are transported using the overlay 1550 nm wavelength. To be cost-effective, the GPON overlay wavelength is split many times and feeds many homes, up to 10000, with the same signal. This is acceptable in current GPON deployments because only broadcast video services are transported on the overlay wavelength, and all narrowcast services, such as internet data and telephony, are transported on the baseband digital signal. In order to use their current infrastructure, the MSO would also transport narrowcast services using RF signals on the overlay wavelength in the forward path. But in this scenario, all homes would share the same narrowcast bandwidth which would severely limit the amount of unique services available for each home.
Further, the MSO's current equipment converts information to be carried over RF signals in the return path. GPON has no option to carry RF signals in the return path.